FIG. 1A is a schematic diagram of an N-bit current-steering digital-to-analog converter. Referring FIG. 1A, the DAC 100 has N current sources 2N-1ILSB, 2N-2ILSB, . . . ILSB, and N switches bN, bN-1, . . . b1. Each of current sources 2N-1ILSB, 2N-2ILSB, . . . ILSB is coupled to one of switches bN, bN-1, . . . b1 respectively. In addition, a resistance RL has a first terminal coupled to a common voltage Vcom, and a second terminal coupled to the N current sources 2N-1ILSB, 2N-2ILSB, . . . ILSB through the switches bN, bN-1, . . . b1. Therefore, when one of the switches turns on, an output current Iout would be generated by the corresponding current sources coupled to the conducted switch.
When one of switches bN, bN-1, . . . b1 turns on, the DAC 100 has a corresponding output resistance Runit, such as shown in FIG. 1B. In addition, in the equivalent circuit of output resistance, the resistance RL would be equivalent to parallel connected resistor Rload and capacitor Cload. Wherein, a ratio of the output resistance Runit and the equivalent resistor Rload could be written as follows.
      R    ratio    =            1      ρ        =                  R        unit                    R        load            Therefore, values of differential nonlinearity (DNL) and integral nonlinearity (INL) could be written as follows.
            DNL      k        ≅                            I          unit                                      (                          1              +                              ρ                ·                k                                      )                    2                    -              I        unit                        INL      k        =                            ∑                      i            =            0                    k                ⁢                  DNL          i                    =                        I          ⁡                      (            k            )                          -                  I          ⁡                      (            0            )                          -                  k          ·                      I            unit                              In addition, a value of spurious-free dynamic range (SFDR) could be written as follows
  SFDR  ≅            (              2        ·                                            R              ratio                        +                          2                              N                -                1                                                          2                          N              -              1                                          )        2  
Since the value of DNL needs to be as small as possible and the value of SFDR needs to be as great as possible, thus ρ would be as small as possible. Therefore, the output resistance Runit needs to be as great as possible.